Rubber, rubber composition, and crosslinked object

ABSTRACT

A nitrile group-containing highly saturated copolymer rubber comprising (a) 0 to 20% by weight of 1,3-butadiene units, (b) 0 to 50% by weight of saturated 1,3-butadiene units, (c) 40 to 50% by weight of α,β-ethylenically unsaturated nitrile monomer units, and (d) 10 to 35% by weight and at least 8% by mol of other monomer units, wherein the sum of 1,3-butadiene units (a) and saturated 1,3-butadiene units (b) is in the range of 20 to 50% by weight. A rubber composition comprising this copolymer rubber and a crosslinking agent gives a crosslinked product exhibiting good oil resistance, namely, good resistance to rancid gasoline and not hardening even when it is placed in contact with oil containing condensed aromatics.

TECHNICAL FIELD

This invention relates to a nitrile group-containing highly saturatedcopolymer rubber used as material for a crosslinked rubber producthaving excellent oil resistance, a crosslinkable rubber compositioncomprising the nitrile group-containing highly saturated copolymerrubber, and a crosslinked product of the crosslinkable rubbercomposition.

BACKGROUND ART

Longer service quality and less maintenance are required forautomobiles, and thus, greater durability is required for automobileparts. Especially, less volume change with time is required for rubberparts placed in contact with oil such as gasoline and a lubricating oil.

As rubber exhibiting less volume change with time when it is placed incontact with oils, a nitrile group-containing highly saturated copolymerrubber having a reduced content of carbon-carbon double bonds, namely, ahydrogenation product of a nitrile group-containing hydrocarbon rubberouch as an acrylonitrile-butadiene copolymer rubber, was proposed, forexample, in Japanese Unexamined Patent Publication No. 854-132647. Acrosslinked product of the nitrile group-containing highly saturatedcopolymer rubber exhibits excellent resistance to rancid gasoline and toozone, and thus, is widely used as sealing materials and hoses inautomobiles. However, the nitrile group-containing highly saturatedcopolymer rubber made by hydrogenating the copolymer tends to hardenwhen it is placed in contact with oils such as gasoline and alubricating oil, and thus, the rubber elasticity does not last for along time. It is not known that a certain hydrogenation rubber productof a nitrile group-containing highly saturated terpolymer rubber havingno tendency of hardening even when it is placed in contact with oils.

Oils such as gasoline and a lubricating oil occasionally contain traceamounts of condensed aromatics having a high boiling point such asα-naphthol, anthracene and phenanthrene, which are residual ingredientsremaining without separation in a step of refining crude oil. It ispresumed that the hardening in oil of a nitrile group-containing highlysaturated copolymer rubber occurs due to the fact that the condensedaromatics hinder movement of a high polymer chain in rubber. However, itis difficult to remove the trace amounts of condensed aromatics in astep of refining crude oil and to produce refined oil products such asgasoline and a lubricating oil, which do not harden the nitrilegroup-containing highly saturated copolymer rubber. Therefore, there hasbeen a demand for an improved nitrile group-containing highly saturatedcopolymer rubber exhibiting reduced tendency of hardening even when itis placed in contact with refined oil products.

A highly saturated copolymer rubber having an improved cold resistanceis known, for example, in Japanese Unexamined Patent Publication No.S63-95242, which is made by hydrogenating a copolymer prepared bycopolymerization of an α,β-ethylenically unsaturated nitrile monomer, aconjugated diene monomer and an unsaturated carboxylic acid monomer. Acrosslinked product of this highly saturated copolymer rubber containsonly a minor amount of nitrile monomer units, and thus, the rubber tendsto exhibits a large change in volume when it is placed in contact withoil, especially gasoline.

DISCLOSURE OF THE INVENTION

A primary object of the present invention is to provide a nitrilegroup-containing highly saturated copolymer rubber which is used asrubber material giving a crosslinked rubber product exhibiting a reducedtendency of hardening and being changed in volume even when it is placedin contact with oils containing residual condensed aromatics, while goodresistance to rancid gasoline is maintained, and further to provide acrosslinkable rubber composition comprising the nitrile group-containinghighly saturated copolymer rubber, and a crosslinked rubber product madefrom the crosslinkable rubber product.

To achieve the above-mentioned object, the present inventors madeextensive research and found that a crosslinked rubber product made bycrosslinking a nitrile group-containing highly saturated copolymerhaving a special monomer unit composition exhibits greatly reducedswelling in oil, excellent resistance to rancid gasoline and greatlyreduced hardening in oil. Based on this finding, the present inventionhas been completed.

Thus, in one aspect of the present invention, there is provided anitrile group-containing highly saturated copolymer rubber comprising(a) 0 to 20% by weight of 1,3-butadiene units, (b) 0 to 50% by weight ofsaturated 1,3-butadiene units, (c) 40 to 50% by weight ofα,β-ethylenically unsaturated nitrile monomer units, and (d) 10 to 35%by weight and at least 8% by mol of other monomer units, wherein the sumof 1,3-butadiene units (a) and saturated 1,3-butadiene units (b) is inthe range of 20 to 50% by weight.

In another aspect of the present invention, there is provided acrosslinkable rubber composition comprising the above-mentioned nitrilegroup-containing highly saturated copolymer rubber and a crosslinkingagent.

In still another aspect of the present invention, there is provided acrosslinked rubber product made by crosslinking the above-mentionedcrosslinkable rubber composition.

BEST MODE FOR CARRYING OUT THE INVENTION Nitrile Group-Containing HighlySaturated Copolymer Rubber

The nitrile group-containing highly saturated copolymer rubber of thepresent invention rubber comprises (a) 0 to 20% by weight of1,3-butadiene units, (b) 0 to 50% by weight of saturated 1,3-butadieneunits, (c) 40 to 50% by weight of α,β-ethylenically unsaturated nitrilemonomer units, and (d) 10 to 35% by weight and at least 8% by mol ofother monomer units, wherein the sum of 1,3-butadiene units (a) andsaturated 1,3-butadiene units (b) is in the range of 20 to 50% byweight.

The content of 1,3-butadiene units (a) in the nitrile group-containinghighly saturated copolymer rubber is in the range of 1 to 20% by weight,preferably 0 to 15% by weight and more preferably 0 to 10% by weight.When the content of 1,3-butadiene units (a) is too large, the resultingcrosslinked rubber product has poor resistance to rancid gasoline andozone and exhibits a large volume change in oil.

The saturated 1,3-butadiene units (b) in the nitrile group-containinghighly saturated copolymer rubber have a structure such that thecarbon-carbon double bonds of 1,3-butadiene units have been saturated byhydrogenation. The nitrile group-containing highly saturated copolymerrubber having saturated 1,3-butadiene units (b) can be made byhydrogenating a nitrile group-containing copolymer rubber having1,3-butadiene units whereby at least part of the 1,3-butadiene units isconverted to saturated 1,3-butadiene units (b). The nitrilegroup-containing highly saturated copolymer rubber having saturated1,3-butadiene units (b) can also be made directly by copolymerizing1-butene with 1,3-butadiene, an α,β-ethylenically unsaturated nitrilemonomer, and other monomer, whereby saturated 1,3-butadiene units areintroduced in polymer. Further, the nitrile group-containing highlysaturated copolymer rubber having saturated 1,3-butadiene units (b) canalso be made by copolymerizing ethylene with 1,3-butadiene, anα,β-ethylenically unsaturated nitrile monomer, and other monomer,whereby units each composed of two adjacent ethylene units are formed aspart of the structural units in polymer.

The content of saturated 1,3-butadiene units (b) in the nitrilegroup-containing highly saturated copolymer rubber is in the range of 0to 50% by weight, preferably 10 to 45% by weight and more preferably 15to 40% by weight. When the content of saturated 1,3-butadiene units (b)is too small, the crosslinked rubber product tends to have poorresistance to rancid gasoline. In contrast, when the content ofsaturated 1,3-butadiene units (b) is too large, the crosslinked rubberproduct exhibits a large volume change in oil.

The sum of 1,3-butadiene units (a) and saturated 1,3-butadiene units (b)in the nitrile group-containing highly saturated copolymer rubber of thepresent invention is in the range of 20 to 50% by weight, preferably 23to 47% by weight and more preferably 25 to 45% by weight. When the sumof 1,3-butadiene units (a) and saturated 1,3-butadiene units (b) is toosmall, the resulting crosslinked rubber product has poor rubberelasticity and is brittle. In contrast, when the sum of 1,3-butadieneunits (a) and saturated 1,3-butadiene units (b) is too large, theresulting crosslinked rubber product exhibits a large volume change inoil.

As specific examples of the α,β-ethylenically unsaturated nitrilemonomer, there can be mentioned acrylonitrile; α-halogenoacrylonitrilesuch as α-chloroacrylonitrile and α-bromoaorylonitrile; andα-alkylacrylonitrile such as mehacrylonitrile and ethacrylonitrile. Ofthese, acrylonitrile is preferable. The α,β-ethylenically unsaturatednitrile monomers may be used either alone or as a combination of atleast two thereof.

The content of α,β-ethylenically unsaturated nitrile monomer units (a)in the nitrile group-containing highly saturated copolymer rubber is inthe range of 40 to 50% by weight, preferably 40 to 48% by weight andmore preferably 41 to 46% by weight. When the content ofα,β-ethylenically unsaturated nitrile monomer unit (c) is too small, theresulting crosslinked rubber product exhibits a large volume change inoil. In contrast, when the content of α,β-ethylenicaliy unsaturatednitrile monomer units (c) is too large the resulting crosslinked rubberproduct has poor rubber elasticity.

The monomer units (d) in the nitrile group-containing highly saturatedcopolymer rubber of the present invention are monomer units other than1,3-butadiene units (a), saturated 1,3-butadiene units (b), andα,β-ethylenically unsaturated nitrile monomer units (c). The monomerunits (d) have been introduced in the nitrile group-containing highlysaturated copolymer rubber by copolymerizing 1,3-butadiene, optional1-butene and an α,β-ethylenically unsaturated nitrile monomer with othercopolymerizable monomer or monomers.

The copolymerizable monomer or monomers include α-olefins other than1-butene, conjugated diene monomers other than 1,3-butadiene,non-conjugated diene monomers, unsaturated carboxylic acid esters,aromatic vinyl monomers, fluorine-containing vinyl monomers, unsaturatedmonocarboxylic acids, and unsaturated polycarboxylic acids andanhydrides thereof.

The α-olefins other than 1-butene include those which have at least 3carbon atoms, preferably not more than 12 carbon atoms. As specificexamples of the α-olefins, there can be mentioned propylene,4-methyl-1-pentene, 1-hexene and 1-octene. If 1-butene is copolymerized,it constitutes saturated 1,3-butadiene units (b) which are regarded asbeing distinguished from monomer units (d).

The conjugated diene monomers other than 1,3-butadiene includes thosewhich have at least 5 carbon atoms and preferably not more than 12carbon atoms. As specific examples of the conjugated diene monomersother than 1,3-butadiene, there can be mentioned isoprene,2,3-dimethyl-1,3-butadiene and 1,3-pentadiene.

As specific examples of the non-conjugated diene monomers, there can bementioned vinylnorbornene, dicyclopentadiene and 1,4-hexadiene.

As specific examples of the α,β-ethylenically unsaturated carboxylicacid esters, there can be mentioned acrylates and methacrylates, whichhave an alkyl group having 1 to 18 carbon atoms, such as methylacrylate, ethyl acrylate, butyl acrylate, n-dodecyl acrylate, methylmethacrylate and ethyl methacrylate; acrylates and methacrylates, whichhave an alkoxyalkyl group having 2 to 18 carbon atoms, such asmethoxymethyl acrylate and methoxyethyl methacrylate; acrylates andmethacrylates, which have a cyanoalkyl group having 2 to 18 carbonatoms, such as α-cyanoethyl acrylate and β-cyanoethyl acrylate;acrylates and methacrylates, which have a hydroxyalkyl group having 2 to18 carbon atoms, such as 2-hydroxyethyl acryalte, hydroxypropyl acrylateand 2-hydroxyethyl methacrylate; acrylates and methaorylates, which havean aminoalkyl group with alkyl groups each having 1 to 19 carbon atoms,such as dimethylaminomethyl acrylate, diethylaminoethyl acrylate anddimethylaminoethyl methacrylate; acrylates and methacrylates, which havea trifluoroalkyl group having 2 to 18 carbon atoms, such astrifluoroethyl acrylate and tetrafluoropropyl methacrylate, benzylacrylates having a fluoro-substituent and benzyl methacrylates having afluoro-substituent, such as fluorobenzyl acrylate and fluorobenzylmethacrylate; and unsaturated dicarboxylic acid monoalkyl esters andunsaturated dicarboxylic acid dialkyl esters, which have an alkyl groupor alkyl groups having 1 to 4 carbon atoms, such as monoethyl maleate,dimethyl maleate, dimethyl fumarate, dimethyl itaconate, n-butylitaconate and diethyl itaconate.

As specific examples of the aromatic vinyl monomers, there can bementioned styrene, α-methylstyrene and vinylpyridine.

As specific examples of the fluorine-containing vinyl monomers, therecan be mentioned fluoroalkyl vinyl ethers having a fluoroalkyl grouphaving 2 to 10 carbon atoms, such as fluoroethyl vinyl ether,fluoropropyl vinyl ether, trifluoromethyl vinyl ether, trifluoroethylvinyl ether, perfluoropropyl vinyl ether and perfluorohexyl vinyl ether;and fluorine-containing vinyl aromatic compounds such as vinylpentafluorobanzoate; and difluroethylene and tetrafluoroethylene.

As specific examples of the unsaturated monocarboxylic acids, there canbe mentioned acrylic acid and methacrylic acid.

As specific examples of the unsaturated polycarboxylic acids, there canbe mentioned itaconic acid, fumaric acid and maleic acid. As specificexamples of the anhydrides of unsaturated polycarboxylic acids, therecan be mentioned itaconic anhydride, fumaric anhydride and maleicanhydride.

The content of monomer units (d) in the nitrile group-containing highlysaturated copolymer rubber is in the range of 10 to 35% by weight,preferably 11 to 33% by weight and more preferably 12 to 30% by weight,and at least 8% by mol, preferably 8 to 30% by mol and more preferably 9to 30% by mol. When the content of monomer units (d) is too small, theresulting crosslinked rubber product tends to harden in oil. Incontrast, when the content of monomer units (d) is too large, theresulting crosslinked rubber product is liable to have greatly reducedmechanical strength.

Ethylene can also be copolymerized. However, when ethylene units arediscretely present in the copolymer chain, the ethylene units aremonomer units (d). When ethylene units form units each composed of twoadjacent ethylene units, the adjacent ethylene units form saturated1,3-butadiene units (b). If an even number of continuous ethylene unitsare present, these ethylene units form a half mol number of saturated1,3-butadiene units. If an odd number of continuous ethylene units arepresent, one of the ethylene units form one monomer unit (d) and theremainder of ethylene units form a half mol number of saturated1,3-butadiene units (b). Thus, if ethylene is copolymerized, it is oftennot clear without analysis whether the resulting copolymer is a nitrilegroup-containing highly saturated copolymer rubber of the presentinvention or not.

As hereinafter mentioned, when a copolymer comprising 1,3-butadieneunits (a), monomer units (c) and monomer units (d) is saturated by, forexample, hydrogenation, at least part of the 1,3-butadiene units (a)becomes saturated to be thereby converted to saturated 1,3-butadieneunits (b), and sometimes a part of monomer units (d) becomes saturated.The latter saturated monomer units are also included in them monomerunits (d).

The sum of monomer units (c) and monomer units (d) is preferably in therange of 50 to 80% by weight, more preferably 53 to 77% by weight andespecially preferably 55 to 75% by weight. When the sum of monomer units(c) and monomer units (d) is too small, the resulting crosslinked rubberproduct exhibits a large volume change in oil. In contrast, when the sumof monomer units (c) and monomer units (d) is too large, the resultingcrosslinked rubber product becomes has poor rubber elasticity andbecomes brittle.

The contents of 1,3-butadiene units (a), saturated 1,3-butadiene units(b), monomer units (c) and monomer units (d) can be determinedadvantageously by employing a combination of plural methods selectedfrom nitrogen content-determination by semi-micro Kjeldahl method,unsaturation content-determination by infrared absorption spectroscopyor iodine value determination, and identification of partial structuresor content ratio determination by infrared absorption spectroscopy,¹H-NMR, ¹³C-NMR and pyrolysis gas chromatography. Of these,identification of partial structures or content ratio determination by¹H-NMR is generally most reliable, but, a plurality of peaks in a ¹H-NMRchart occasionally coincide with each other which render thedetermination difficult. Therefore, a combination of ¹H-NMR with othermethods is especially preferable.

The nitrile group-containing highly saturated copolymer rubber of thepresent invention preferably has a number average molecular weight inthe range of 10,000 to 2,000,000, more preferably 30,000 to 1,500,000and especially preferably 50,000 to 1,000,000. When the number averagemolecular weight is too small, the rubber tends to have too lowviscosity and have poor mechanical strength such as tensile strength. Incontrast, when the number average molecular weight is too large, therubber tends to have too high viscosity and have poor processability.

The process for producing the nitrile group-containing highly saturatedcopolymer rubber of the present invention is not particularly limited,but preferable is a process wherein a copolymer of 1,3-butadiene, anα,β-ethylenically unsaturated nitrile monomer and other copolymerizablemonomer or monomers is hydrogenated whereby at least part of unsaturatedbonds in the backbone chain is saturated. For example, a conventionalprocess as described in Japanese Unexamined Patent Publication No.H8-100025 can be employed wherein the copolymer is made by an emulsionpolymerization procedure and then the copolymer is hydrogenated.

Crosslikable Rubber Composition

The crosslinkable rubber composition of the present invention comprisesas essential ingredients the above-mentioned nitrile group-containinghighly saturated copolymer rubber and a crosslinking agent, and otheringredients as optional ingredients.

The crosslinking agent is not particularly limited provided that it iscapable of crosslinking the nitrile group-containing highly saturatedcopolymer rubber of the present invention. However, a sulfur-containingcrosslinking agent and an organic peroxide crosslinking agent arepreferably used.

As specific examples of the sulfur-containing crosslinking agent, therecan be mentioned sulfur such as powdery sulfur and precipitated sulfur;and organic sulfur compounds such as 4,4′-dithiomorpholine, ttramethylthiuram disulfide, tetraethylthiuram disulfide and highmolecular weight polysulfide. The amount of the sulfur-containingcrosslinking agent is in the range of 0.1 to 10 parts by weight,preferably 0.2 to 7 parts by weight and more preferably 0.3 to 5 partsby weight, based on 100 parts by weight of the nitrile group-containinghighly saturated copolymer rubber. When the amount of thesulfur-containing crosslinking agent is too small, the crosslinkingdensity of rubber is reduced and the permanent set becomes large. Incontrast, when the amount of the sulfur-containing crosslinking agent istoo large, the resulting crosslinked rubber is liable to have poorflexural fatigue resistance and high dynamic heat build-up.

The organic peroxide crosslinking agent includes dialkyl peroxides,diacyl peroxides and peroxyesters. As specific examples of the organicperoxide crosslinking agent, there can be mentioned dialkyl peroxidessuch as dicumyl peroxide, di-tert-butyl peroxide,2,5-dimethyl-2,5-di(tert-butyl-peroxy)-3-hexyne,2,5-dimethyl-2,5-di(tert-butyl-peroxy)hexane and1,3-bis(tert-butyl-peroxyisopropyl)benzene; diacyl peroxides such asbenzoyl peroxide and isobutyryl peroxide; and peroxy esters such as2,5-dimethyl-2,5-bis(benzoyl-peroxy)hexane andtert-butyl-peroxyisopropyl carbonate. The amount of the organic peroxidecrosslinking agent is in the range of 0.5 to 8 parts by weight,preferably 0.5 to 7 parts by weight and more preferably 0.5 to 5 partsby weight, based on 100 parts by weight of the nitrile group-containinghighly saturated copolymer rubber. When the amount of the organicperoxide crosslinking agent is too small, the crosslinking density ofrubber is reduced and the permanent set becomes large. In contrast, whenthe amount of the organic peroxide crosslinking agent is too large, theresulting crosslinked rubber is liable to have poor rubber elasticity.

The crosslinking agent can be used either alone or as a combination ofat least two thereof. The crosslinking agent can be used as a dispersionin clay, calcium carbonate or silica whereby processability is enhanced.

A crosslinking accelerator can be used in combination with acrosslinking agent. The crosslinking accelerator is not particularlylimited. The crosslinking accelerator used in combination with asulfur-containing crosslinking agent includes zinc oxide, thiuramcrosslinking accelerators, guanidine crosslinking accelerators,sulfenamide crosslinking accelerators, thiazole crosslinkingaccelerators and dithiocarbamate crosslinking accelerators. As specificexamples of the crosslinking accelerator, there can be mentioned thiuramcrosslinking accelerators such as tetramethylthiuram disulfide,tetraethylthiuram disulfide and N,N′-dimethyl-N,N′-diphenylthiuramdisulfide; guanidine crosslinking accelerators such asdiphenylguanidine, diorthotolylguanidine and orthotolylbiguanide;sulfenamide crosslinking accelerators such asN-cyolohexyl-2-benzothiazylsulfenamide,N,N′-diisopropyl-2-benzothiazylsulfenamide andN-tertiary-butyl-2-benzothizylsulfenamide; thiazole crosslinkingaccelerators such as 2-mercaptobenzothiazole and dibenzothizyldisulfide; and dithiocarbamate crosslinking accelerators such astellurium dimethyldithiocarbamate and zinc dimethyldithiocarbamate

As specific examples of the crosslinking accelerator used in combinationwith an organic peroxide crosslinking agent, there can be mentionedtrimethylolpropane trimethacrylate, N,N′-m-phenylenedimaleimide,triallyl isocyanurate, polyfunctional methacrylate monomers, acrylicacid metal salts and methacrylic acid metal salts. These crosslinkingaccelerators may be used as a combination of at least two thereofdepending upon the particularly use of rubber.

According to the need, various ingredients can be incorporated providedthat the effect of the present invention can be achieved. Theingredients include, for example, a reinforcing agent, a filler, an ageresister, an antioxidant, a light stabilizer, a scorch retarder, aplasticizer, a processing aid, a lubricant, an adhesive mass, a slipingagent, a flame retardant, a mildew proofing agent, an antistatic agent,and a colorant.

Various rubbers, elastomers and resins can be incorporated unless thecrosslinkable rubber composition of the present invention issubstantially injured. As specific examples of such ingredients, therecan be mentioned natural rubber, polybutadiene rubber, polyisoprenerubber, acrylic rubber, styrene-butadiene rubber,acrylonitrile-butadiene rubber, chloroprene rubber, olefin elastomer,styrene elastomer, vinyl chloride elastomer, polyester elastomer,polyamide elastomer, polyurethane elastomer and polysiloxane elastomer.Crosslinking agents and crosslinking accelerators for crosslinking therubbers, elastomers and resins may be incorporated unless thecrosslinkable rubber composition of the present invention issubstantially injured.

The crosslinkable rubber composition of the present invention can beprepared by mixing together the rubber, the crosslinking agent andoptional ingredients by employing an appropriate mixing method such asroll mixing, Banbury mixing, screw mixing or solution mixing. The orderof mixing is not particularly limited. Preferably, ingredients which arenot easily thermally decomposed are first thoroughly incorporated in therubber, and then, crosslinking agent and crosslinking accelerator, whichare readily thermally decomposed, are added at a temperature lower thanthe crosslinking initiating temperature for a time as short as possible.

Crosslinked Rubber Product

The crosslinked rubber product of the present invention is made bycrosslinking the above-mentioned crosslinkable rubber composition.

The method for making the crosslinked rubber product is not particularlylimited. A method wherein shaping and crosslinking are simultaneouslycarried out or a method wherein shaping is first carried out and thencrosslinking is carried out may be employed. An appropriate method ischosen depending upon the particular shape of crosslinked product orother factors. The method of simultaneously carrying out shaping andcrosslinking includes, for example, compression molding, transfermolding and injection molding. The method of shaping followed bycrosslinking includes, for example, a method wherein a rubbercomposition is subjected to extrusion shaping and then crosslinked in avulcanizing pan.

The crosslinkable rubber composition of the present invention iscrosslinked by heating. The heating temperature is preferably in therange of 130 to 210° C., more preferably 140 to 200° C. When the heatingtemperature is too low, a substantially long time is required forcrosslinking and the crosslinking density is liable to be reduced. Incontrast, when the heating temperature is too high, the crosslinkingtime is too short and a defective molding is liable to be produced.

After crosslinking, i.e., first crosslinking, the crosslinked rubberproduct may be subjected to second crosslinking. The crosslinking timefor the first crosslinking and the second crosslinking can be chosen onthe crosslinking method, crosslinking temperature and shape of therubber product. Usually the crosslinking time is chosen in the range ofone minute to 20 hours depending on the crosslinking density and theproduction efficiency.

The heating means may be appropriately chosen from those which areemployed for crosslinking rubbers and which includes, for example,press-heating, steam-heating, oven-heating and hot air-heating.

Now the invention will be specifically described by the followingexamples and comparative examples wherein parts and % are by weightunless otherwise specified. The properties of rubber were evaluated bythe following methods.

(1) Tensile Strength and Breaking Elongation

A crosslinked rubber composition was shaped into a sheet with athickness of 2 mm. The sheet was die-cut by a #3 dumbbell die to preparea specimen. Breaking tensile strength and breaking elongation weremeasured according to JIS K6251 at a grip separation speed of 500mm/min. The smaller the tensile strength, and the smaller theelongation, the rubber product has better fitting property.

(2) Hardness

A crosslinked rubber composition was shaped into a sheet with athickness of 2 mm. The sheet was die-cut to prepare a square specimenhaving a size of 25 mm×15 mm. Hardness was measured according to JISK6253 by using IRHD micro-hardness tester.

(3) Volume Change

A crosslinked rubber composition was shaped into a sheet with athickness of 2 mm. The sheet was die-cut to prepare a square specimenhaving a size of 25 mm×15 mm. The specimen was immersed in Fuel D (mixedliquid of isooctane/toluene=3/2 by volume) at 40° C. for 140 hours. Thevolume change was measured according to JIS K6258.

(4) Hardening

A crosslinked rubber composition was shaped into a sheet with athickness of 2 mm. The sheet was die-cut to prepare a square specimenhaving a size of 25 mm×15 mm. The specimen was immersed in mixed Fuel Dliquid at 40° C. for 10 days. The mixed Fuel D liquid was prepared byincorporating a condensed aromatic mixture ofanthracene/phenanthrene=1/1 by weight in Fuel D to give an oilcontaining 2% of the condensed aromatic mixture. The specimen was thenvacuum-dried for 48 hours and its hardness was measured. For comparison,the specimen was immersed in Fuel D at 40° C. for 10 days, and thenVacuum-dried for 48 hours and its hardness was measured. The hardeningwas expressed by the difference between the hardness as measured afterimmersion in Fuel D+condensed aromatic mixture, and the hardness asmeasured after immersion in Fuel D alone. When the hardness differenceis 5 points or larger, the hardening is regarded as occurred.

(5) Resistance to Rancid Gasoline

A crosslinked rubber composition was shaped into a sheet with athickness of 2 mm. The sheet was die-cut to prepare a square specimenhaving a size of 25 mm×15 mm. The specimen was immersed in a test oil at40° C. for 240 hours. The test oil was prepared by incorporating 3% oflauroyl peroxide into Fuel C (mixed liquid of isooctane/toluene=1/1 byvolume). After immersion, the specimen was vacuum dried at 23° C. for 24hours. The dried specimen was folded by 180° and occurrence of cracks onthe outer surface of folded specimen was observed by the naked eye. Whenoccurrence of cracks is observed, the resistance to rancid gasoline isregarded as poor.

EXAMPLES 1-7, COMPARATIVE EXAMPLES 1-7

100 parts of a nitrile group-containing highly saturated rubber having amonomer composition shown in Table 1 or Table 2 was kneaded togetherwith 30 parts of carbon black N762 (“Asahi #50” available from AsahiCarbon K.K., average particle diameter: about 80 nm, specific surfacearea: about 23 m²/g), and 1 part of stearic acid in a closed mixerhaving a jacket temperature of 50° C. Using an open roll having asurface temperature of 50° C., 1.5 parts of tetraethylthiuram disulfide(“Nocceler TET” available from Ouchi Shinko K.K.) and 1.5 parts ofN-cyolohexyl-2-benzothiazylsulfenamide (“Nocceler CZ” available fromOuchi Shinko K.K.) were simultaneously added to the kneaded rubber. Then0.5 part of sulfur (available from Hosoi kagaku Kogyo K.K., 325 mesh)and 5 parts of zinc oxide (available from Seido Kagaku Kogyo K.K.,purity: higher than 99.5) were added in this order. The thus-obtainedrubber composition was press-molded at 170° C. and 9 MPa for 20 minutesto prepare a specimen. The physical properties of the specimen wereevaluated. The results are shown in Table 1 and Table 2.

EXAMPLE 8

100 parts of a nitrile group-containing highly saturated rubber having amonomer composition shown in Table 1 was kneaded together with 50 partsof carbon black N550 (“Asahi #50” available from Asahi Carbon K.K.,average particle diameter: about 45 nm, specific surface area: about 40m²/g), 1 part of substituted diphenylamine (“Naugard 445” available fromUniroyal Chemicals Co., and 1 part of 2-mercaptobenzothiazole zinc sdalt(“Nocrac NBZ” available from Ouchi Shinko K.K.) in a closed mixer havinga jacket temperature of 50° C. Using an open roll having a surfacetemperature of 50° C., 1,3-bis(tert-butyl-peroxyisopropyl)benzene(“Vulcup 40KE” available from Hercules Co.) was added to the kneadedrubber. The thus-obtained rubber composition was press-molded at 170° C.and 9 MPa for 20 minutes to prepare a specimen. The physical propertiesof the specimen were evaluated. The results are shown in Table 1.

The contents of monomer units in the nitrile group-containing highlysaturated copolymer rubber were measured by ¹H-NMR, iodine valuedetermination and nitrogen content determination by semi-micro Kjeldahlmethod. It was confirmed that there was no contradiction between theamount of monomers consumed for polymerization and the amount ofresidual monomers.

TABLE 1 Examples 1 2 3 4 5 6 7 8 Monomer unit composition of nitrilegroup-containing highly saturated copolymer rubber (a) 1,3-Butadieneunits (wt. %)  7  7  5  5  6  6  6  6 (b) Hydrogenated butadiene units(wt. %) 28 19 29 32 38 34 34 34 (c) Acrylonitrile units (wt. %) 44 44 4542 44 42 42 42 (d) Butyl acrylate units (wt. %) 21 30 21 21 — — — —Isoprene units (wt. %) — — — — 12 18 — — Styrene units (wt. %) — — — — —— 18 18 Monomer units (d) (mol %) 10 15 10 10 10 15 10 10 Monomer units(a) + (b) (wt. %) 35 26 34 37 44 40 40 40 Number average molecularweight (×10³) 90 91 89 91 88 90 90 97 Dry physical properties Tensilestrength (MPa) 17 15 18 19 21 20 22 23 Elongation (%) 480  460  480 450  460  480  460  350  Hardness (IRHD-micro) 65 63 63 62 62 63 62 78Fuel D immersion test Volume change 32 31 35 38 38 42 42 43 Hardeningtest Difference in hardness  0  0  2  2  2  0  0  0 Ranoid gasolineresistance test Presence of surface cracks not not not not not not notnot

The content of isoprene units includes contents of isoprene units andsaturated isoprene units. The content of saturated conjugated dieneunits, namely, the contents of saturated 1,3-butadiene units andsaturated isoprene units were determined by measuring the ratio thereofby ¹H-NMR and by measuring the amount of unsaturated bonds remaining inthe backbone chain.

TABLE 2 Comparative Examples 1 2 3 4 5 6 Monomer unit composition ofnitrile group-containing highly saturated copolymer rubber (a)1,3-Butadiene units  6  6  7  5  6  8 (wt. %) (b) Hydrogenated butadiene50 47 41 50  9 39 units (wt. %) (c) Acrylonitrile units (wt. %) 44 44 4425 50 38 (d) Butyl acrylate units —  3  8 20 35 15 (wt. %) Monomerunits(d) (mol %)  0  1  4  9 18  7 Monomer units(a) + (b) 56 53 48 55 1547 (wt. %) Number average molecular 87 92 91 89 90 92 weight (×10³) Dryphysical properties Tensile strength (MPa) 27 25 20 17 10 23 Elongation(%) 580  520  510  510  430  560  Hardness (IRHD-micro) 63 62 61 60 6860 Fuel D immersion test Volume change 35 35 36 70 25 48 Hardening testDifference in hardness 20 17  7  0  0  8 Rancid gasoline resistance testPresence of surface cracks not not not not not not

A crosslinked product of the nitrile group-containing highly saturatedcopolymer rubber of the present invention harm appropriate tensilestrength, elongation and hardness, exhibits reduced volume change inoil, does not harden in oil, and has excellent resistance to rancidgasoline.

In contrast, a crosslinked product (Comparative Example 1) of nitrilegroup-containing highly saturated copolymer rubber containing largeamounts of monomer units (a) and monomer units (b) but not containingmonomer units (d), a crosslinked product (Comparative Example 2, 3) ofnitrile group-containing highly saturated copolymer rubber containinglarge amounts of monomer units (a) and monomer units (b) but containinga reduced amount of monomer units (d), and a crosslinked product(Comparative Example 6) of nitrile group-containing highly saturatedcopolymer rubber containing monomer units (d) in a sufficient weight %but reduced mol % poor have a large elongation and tends to harden inoil.

A crosslinked product (Comparative Example 4) of a nitrilegroup-containing copolymer rubber containing a reduced amount of monomerunits (c) has a large elongation and exhibits a large volume change inoil. A crosslinked product (Comparative Example 5) of a nitrilegroup-containing copolymer rubber containing too large amounts ofmonomer units (c) and monomer units (d) has poor tensile strength.

A crosslinked product of the nitrile group-containing highly saturatedcopolymer rubber of the present invention exhibits good resistance torancid gasoline, a reduced volume change in oil and does not harden evenwhen it is placed in contact with oil containing condensed aromatics,and good dry physical properties.

INDUSTRIAL APPLICABILITY

A crosslinked rubber product of the present invention is useful anmaterials for sealing material, cushioning material, protectivematerial, wire coating material, and materials for belts, hoses,diaphragms, boots and rolls, used in automobiles and other transports,general instruments and apparatuses, electrical and electronicinstruments and buildings. Especially the crosslinked rubber product issuitable for materials for fuel hoses and sealing materials.

What is claimed is:
 1. A nitrile group-containing highly saturatedcopolymer rubber comprising (a) 0 to 20% by weight of 1,3-butadieneunits, (b) 10 to 45% by weight of saturated 1,3-butadiene units, (c) 41to 46% by weight of α,β-ethylenically unsaturated nitrile monomer units,and (d) 10 to 35% by weight and at least 8% by mol of other monomerunits, wherein the sum of 1,3-butadiene units (a) and saturated1,3-butadiene units (b) is in the range of 20 to 50% by weight.
 2. Thenitrile group-containing highly saturated copolymer rubber according toclaim 1, which comprises (a) 0 to 15% by weight of 1,3-butadiene units,(b) 10 to 45% by weight of saturated 1,3-butadiene units, (c) 41 to 46%by weight of α,β-ethylenically unsaturated nitrile monomer units, and(d) 11 to 33% by weight and 8 to 30% by mol of other monomer units,wherein the sum of 1,3-butadiene units (a) and saturated 1,3-butadieneunits (b) is in the range of 23 to 47% by weight.
 3. The nitrilegroup-containing highly saturated copolymer rubber according to claim 2,which comprises (a) 0 to 10% by weight of 1,3-butadiene units, (b) 15 to40% by weight of saturated 1,3-butadiene units, (c) 41 to 46% by weightof α,β-ethylenically unsaturated nitrile monomer units, and (d) 12 to30% by weight and 9 to 30% by mol of other monomer units, wherein thesum of 1,3-butadiene units (a) and saturated 1,3-butadiene units (b) isin the range of 25 to 30% by weight.
 4. The nitrile group-containinghighly saturated copolymer rubber according to claim 1, wherein the sumof monomer units (c) and monomer units (d) is in the range of 50 to 80%by weight.
 5. The nitrile group-containing highly saturated copolymerrubber according to claim 1, wherein the sum of monomer units (c) andmonomer units (d) is in the range of 55 to 75% by weight.
 6. The nitrilegroup-containing highly saturated copolymer rubber according to claim 1,which is a product prepared by hydrogenating a copolymer of1,3-butadiene, an α,β-ethylenically unsaturated nitrile monomer, and amonomer copolymerizable with 1,3-butadiene and the α,β-ethylenicallyunsaturated nitrile monomer.
 7. The nitrite group-containing highlysaturated copolymer rubber according to claim 1, whereinα,β-ethylenically unsaturated nitrite monomer is selected fromacrylonitrile, α-chloroacrylonitrile, α-bromoacrylonitrile,methacrylonitrile and ethacrylonitrile.
 8. The nitrile group-containinghighly saturated copolymer rubber according to claim 1, which has anumber average molecular weight of 10,000 to 2,000,000.
 9. Acrosslinkable rubber composition comprising 100 parts by weight of thenitrile group-containing highly saturated copolymer rubber as claimed inclaim 1, and 0.1 part to 10 parts by weight of a sulfur-containingcrosslinking agent or 0.5 part to 8 parts by weight of an organicperoxide crosslinking agent.
 10. A crosslinked rubber product made bycrosslinking the crosslinkable rubber composition as claimed in claim 9.11. The nitrile group-containing highly saturated copolymer rubberaccording to claim 1, wherein said other monomer units are units derivedfrom at least one monomer selected from the group consisting ofα-olefins other than 1-butene, conjugated diene monomers other than1,3-butadiene, non-conjugated diene monomers, unsaturated carboxylicacid esters, aromatic vinyl monomers, fluorine-containing vinylmonomers, unsaturated monocarboxylic acids, and unsaturatedpolycarboylic acids and anhydrides thereof.
 12. The nitrilegroup-containing highly saturate copolymer rubber according to claim 1,wherein said other monomer units are units derived from at least onemonomer selected from the group consisting of butyl acrylate, isopreneand styrene.
 13. The nitrile group-containing highly saturated copolymerrubber according to claim 1, wherein said other monomer units are unitsderived from at least one monomer selected from the group consisting ofisoprene and styrene.